1. Field of the Invention
This invention relates to a building low yield ratio hot-dip galvanized cold rolled steel sheet having an improved refractory property and used for various building structures in the fields of pre-fab building material, civil and marine structures, and a method of producing same.
2. Description of the Related Art
The refractory property of a building structure is important, and various countermeasures to the refractory property are effected in all buildings, whether large or small. Nevertheless, as described in the Japanese unexamined patent publication (Kokai) No. 63-47451, countermeasures against fire are actually effected by using a refractory coating, and thus the building costs are increased and the utilizable space for a building structure is reduced.
Recently, the refractory designs were examined and a new Refractory Design Law for Building Structures was established in 1987. At that time, a conventional requirement for a tolerance of the steel material to a temperature of a fire, i.e., 350.degree. C. or less, was removed, and thus the ability of a refractory coating can be determined by a high temperature strength of a steel sheet at a load actually effected on a building structure, and when a high temperature strength of a steel sheet material can be ensured, the use of the steel sheet without such a coating becomes possible.
Recently, the present inventors examined the technology for a Cu-added refractory building-hot rolled steel sheet, and a refractory building hot-dip galvanized hot rolled steel sheet, and filed Japanese patent application Nos. 1-26225 and 1-16446, respectively, for these inventions.
It is well know that, as a roof material or a pre-fab building material of a building structure, a cold rolled steel sheet or a cold rolled steel hoop is used as a raw material.
The present inventor filed, as a patent application relating to a technology of a refractory building cold rolled steel sheet, Japanese Patent Application No. 1-27297.
A corrosion resistance is also necessary for a building steel, and thus a technology of galvanizing a cold rolled steel sheet by a continuous hot-dip galvanizing process having an improved productivity and reduced costs is required.
When the galvanizing of a cold rolled steel hoop or steel sheet is effected by using the continuous hot-dip galvanizing process, the feeding speed cannot be effectively lowered because of the needs of mass production. Further, to recrystallize and obtain an improved plating adhesion, the reducing temperature cannot be greatly lowered. Further in this process, a quenching process is required after an annealing.
It is difficult to ensure the high temperature strength of a conventional steel because of a crystal grain growth and coarsening of a precipitate carbide solution, etc. Further, although a high alloy heat resisting metal exists as an Fe system, a Ni system and a Ti system, this high alloy heat resisting metal is too expensive for use as a material in a large amount for a building.